This invention relates in general to mass transfer columns and, more particularly, to downcomers associated with vapor-liquid contact trays placed within the columns. The invention also relates to methods for using the trays to effect mass transfer between vapor and liquid streams flowing within the column.
Vapor-liquid contact trays are used in mass transfer columns to facilitate interaction and mass transfer between vapor and liquid streams flowing through the column. The trays typically have a tray deck with liquid inlet and outlet ends and an opening formed at the outlet end of the tray deck. A downcomer is positioned at the opening in the tray deck and provides a passage for removing liquid from the outlet end of the tray deck and directing it downwardly to the inlet end of the underlying tray deck. The liquid then flows across the underlying tray and enters the downcomer at the outlet end of that tray deck for passage to the next underlying tray. This pattern is then repeated on each underlying tray.
As liquid is flowing across the tray deck on these vapor-liquid contact trays, vapor passes upwardly through apertures provided in the "active area" of the tray deck and interacts with the liquid to form a frothy two-phase mixture. Most of the vapor then disengages from the mixture and passes upwardly through the apertures in the overlying tray deck. A portion of the vapor, however, remains entrained with the liquid entering the downcomer and passes downwardly to the underlying tray. If this vapor cannot be separated from the liquid in the downcomer, it will limit the liquid handling capacity of the downcomer. Return of vapor to the underlying tray is also generally undesirable in that it limits the mass transfer efficiency of the tray.
The efficiency of a tray can also be reduced by liquid "weeping" or passing downwardly through the vapor apertures in the tray deck rather than flowing completely across the tray deck and interacting with the ascending vapor. Weeping would be particularly problematic in the inlet area of the tray deck underlying the downcomer discharge outlet because the downward force of the liquid exiting the downcomer would force the liquid through the vapor apertures. In order to reduce weeping in this inlet area, apertures are typically omitted from that portion of the tray deck.
One disadvantage to eliminating the vapor apertures from the inlet area of the tray deck is the active area of the tray is reduced, resulting in reduced tray capacity. A number of tray modifications have been utilized or proposed in order to reduce or overcome this disadvantage, including using a sloping downcomer wall to reduce the horizontal cross-sectional area of the downcomer discharge outlet, thereby reducing the size of the inlet area and increasing the active area of the tray deck. However, if the discharge outlet is sized too small, liquid may back up and flood the downcomer, thereby limiting the liquid handling capacity of the downcomer.
In addition to the size of the discharge opening, there are other factors that can affect the liquid handling capacity of the downcomer. One of these factors is known as the downcomer clearance and is the vertical spacing between the downcomer outlet and the top surface of the underlying tray deck. Increases in the downcomer clearance generally result in increases in the liquid capacity of the downcomer. In applications where high liquid flow rates are encountered, it is often desirable to have the downcomer outlet positioned above rather than below the height of the weir or the liquid level on the underlying tray so that liquid can more easily exit the downcomer.
One approach to increasing the liquid handling capacity of a downcomer is disclosed in U.S. Pat. No. 5,213,719 to Chuang. In that patent, a two-stage downcomer consisting of an upstream downcomer and an adjacent downstream downcomer are positioned at the opening in the tray deck. A weir positioned between the downcomers forces liquid to fill the upstream downcomer before it rises above the level of the weir and enters the downstream downcomer. At higher liquid flow rates, this two-stage downcomer was said to accommodate liquid flow rates up to 80% greater than through a single downcomer. One potential disadvantage to this type of downcomer, however, is a partition wall separates the upstream downcomer from the downstream downcomer and liquid entering either downcomer is blocked by the partition wall from passing into the other downcomer. As a result, essentially no liquid is presented to the downstream downcomer until liquid flow rates are high enough to cause liquid to fill the upstream downcomer and accumulate on the tray deck to a sufficient level to overflow the weir. The outlet of the downstream downcomer must thus extend significantly below the liquid level on the underlying tray to seal against vapor bypassing the active area of the tray deck by entering and traveling upwardly through the downstream downcomer. As mentioned above, this reduction in downcomer clearance can reduce the capacity of the downcomer, particularly under high liquid rate conditions. In addition, the liquid discharge from the upstream downcomer is directed vertically downward onto the active area of the tray and can weep through the vapor apertures, thereby bypassing interaction with vapor on the tray deck and reducing the efficiency of the tray. As a result, a need has developed for a high capacity downcomer that does not require that the downcomer discharge outlet be positioned below the weir height on the underlying tray and/or has a lower incidence of liquid weeping.